Manage your subscription

Acoustic ‘superlens’ could mean finer ultrasound scans

By Robert Adler

Materials that bend sound waves backwards could be used to make perfect sound proofing and sharper medical scans, say UK-based researchers.

In the past few years, scientists have learned how to create exotic “metamaterials” that control light and other electromagnetic radiation in surprising ways. These materials have resulted in developments such as invisibility cloaks and superlenses that can break the rules that limit the power of normal lenses.

A new study proves the same principal works for sound waves and provides a blueprint for material needed to do so. Devices based on such acoustic metamaterials could produce ultra-sharp medical scans, more detailed seismic maps, and even earthquake-resistant buildings.

The properties of metamaterials stem from their structure rather than their composition. A team led by Sébastien Guenneau, a mathematician at the University of Liverpool, in the UK, designed a metamaterial for sound waves and modelled how it would work.

Advertisement

It is made from an array of doughnut-shaped voids in a silica sheet. Each donut is split to form a pair of C’s facing each other, with rigid bars running above and below each pair.

Negative refraction

Using Newtonian mechanics, the researchers showed that the interaction between properly sized and spaced voids and bars would produce a negative refractive index for particular sound waves. Effectively, it would bend waves in the opposite direction to a normal material.

“We show that using feasible materials structured in a very specific way we can achieve negative refraction,” says Guenneau.

One possible application for the material is using a flat sheet of it as a “superlens” to produce higher resolution images from existing ultrasound probes.

Conventional lenses cannot focus on anything smaller than roughly half the wavelength of light or sound they use. That is because they cannot recover and focus weak scattered waves that are needed to “see” the really small features.

Thanks to their odd refractive properties, though, metamaterial superlenses can – so the acoustic superlens designed by Guennea and colleagues could focus on details that are invisible to today’s equipment.

Such a superlens could be used at a variety of scales. “You can build one to see a foetus,” says Guenneau, “or something underground”. Imaging underground features requires long-wavelength sound that consequently gives poor resolution, an acoustic superlens could help.

Earthquake protection

Guenneau’s group also showed that a checkerboard array of their halved ring-shapes can stop sound waves passing. This pattern could be used to damp noise or vibrations on a wide range of scales, from tiny electronic or mechanical components to large buildings protected from the earthquake shear waves using columns with the metamaterial incorporated within.

Although the work is so far mathematical, the team is confident its acoustic metamaterial can be built. The model is based on the known properties of silica and and the voids cut into it are accompanied by stiff bars with properties similar to carbon nanotubes.

“The main challenge is to be able to draw this letter C, repeated periodically, at the micron scale,” says Guenneau.

Chiara Daraio who studies highly nonlinear acoustic metamaterials at Caltech, in Pasadena, California, US, agrees that Guenneau’s group has come up with a design for a novel and useful metamaterial. “They have a very new approach to perfect lenses in acoustics,” she says.

“From an experimental perspective, it seems challenging but feasible. It made me wish I could run an experiment tomorrow,” Daraio adds.